This invention relates to the field of imaging devices, and in particular to those devices containing a sensor array. The invention is most particularly applicable to an amorphous silicon X-ray image sensor array usable for both fluoroscopic and radiographic imaging operations.
Two-dimensional sensor arrays are well-known devices for real time imaging of incident high energy radiation (see R. A. Street et al., xe2x80x9cLarge Area Image Sensor Arraysxe2x80x9d, in Technology and Applications of Amorphous Silicon, Editor R. A. Street, Springer Series in Materials Science 37, Springer-Verlag , Berlin, 2000, chapter 4, p. 147, for a general description of the structure of the arrays). Such sensor arrays are particularly advantageous for X-ray imaging because they present a relatively large size image sensor array. Each sensor operates on the principal of integrating a charge representative of the quantities of ionizing radiation incident on the sensor. In the direct detection approach, incident high-energy radiation (e.g., X-ray photons) is directly converted to a charge by the sensor. In the indirect detection approach, a phosphor converter absorbs high energy radiation (e.g., X-ray photons) and generates a proportional amount of visible light that is then converted to a charge by the sensor.
Despite the development of recent medical imaging modalities, such as computed tomography (CT), ultrasound, nuclear medicine and magnetic resonance imaging (MRI), all of which are digital, X-ray imaging systems remain an important tool for medical diagnosis. Although the majority of X-ray imaging systems in current use are of analog design, digital radiology is an area of considerable recent growth. Digital radiology provides significant advantages over its analog counterpart, such as easy comparison of radiological images with those obtained from other imaging modalities, the ability to provide image networking within a hospital for remote access and archiving, facilitating computer aided diagnosis by radiologists, and facilitating teleradiology (i.e., remote diagnostic service to poorly populated regions from a central facility).
Digital radiology provides two dominant modes of imaging that have different operational characteristics. In radiography, a single image frame is acquired under conditions of a large signal (e.g., using a relatively high X-ray dose). In fluoroscopy, frames are acquired at typically 30 or 60 frames per second and the signal (e.g., X-ray dose) is small, at least 10-100 times lower than that used in radiography.
It is highly desirable for an imaging device to perform both radiography and fluoroscopy functions. Apart from the cost benefits associated with a single imager that performs both radiography and fluoroscopy functions, such an imager is needed by the radiologist to rapidly switch between the low contrast, low dose fluoroscopic mode and a radiographic mode to obtain high contrast images. It is also highly desirable to perform various fluoroscopy operations using various intermediate intensities (i.e., between the conventional fluoroscopic and radiographic intensities).
While image sensors utilized to perform fluoroscopic and radiographic operations are generally reasonably compatible, there are some conflicts in the design approach, particularly concerning the pixel capacitance that stores the image charge and also generates electronic noise. The saturation signal for an image sensor pixel is given by CSVMAX, where CS is the capacitance of the pixel and VMAX is the maximum voltage to which the capacitance can be charged. For a typical sensor array that uses an amorphous silicon (a-Si:H) photodiode as the pixel sensor, the capacitance is 1-2 pF for a pixel size in the 100-200 micron range using a 5 V bias voltage. The maximum pixel charge of 5-10 pC is comfortably enough for radiography but greatly exceeds the requirements for fluoroscopy.
Large pixel capacitance would not be a problem for fluoroscopy except that it contributes to electronic noise. There are many contributions to the noise of an imager, but the one that is ultimately impossible to overcome with present array designs is the kTC noise of the pixel, which arises from the thermal noise of the resistance of the switching transistor, and whose magnitude is entirely determined by the pixel capacitance. Therefore, it is not possible to efficiently utilize a conventional pixel for both fluoroscopic and radiographic imaging because the large capacitance needed to perform radiography produces too much noise when the same pixel is utilized for fluoroscopy.
What is needed is a single image sensor that can be optimized for both fluoroscopic and radiographic operating modes.
The present invention is directed to an imaging apparatus in which each pixel of a sensor array includes one or more capacitor loads that are selectively coupled to or decoupled from the usual sensor capacitance to facilitate multi-mode operation. This selectable capacitor load arrangement provides a method for reducing the electronic noise in, for example, a-Si:H medical imagers during fluoroscopy operation, while maintaining the high dynamic range required for radiography.
Each pixel circuit of the imaging apparatus includes a sensor connected between a bias voltage and a first transistor, which is controlled by a gate line to pass collected image data to a data line, and one or more capacitor circuits that include the selectable capacitor load and mode control transistor connected in series between the sensor and ground. The mode control transistors are controlled by a global enable signal that determines the operating mode of image sensor array. When a global enable signal is asserted (e.g., during a radiographic operating mode), a selected mode control transistor of each pixel is turned on, thereby coupling the associated additional capacitor load to the sensor (i.e., effectively increasing the total capacitance) of each pixel. When the selected global enable signal is subsequently de-asserted (e.g., during a fluoroscopic operating mode), the associated mode control transistor of each pixel is turned off, thereby isolating the additional capacitor load from the sensor (i.e., effectively decreasing electronic noise). Accordingly, a single sensor array is provided that is selectively optimized for multi-mode operations.
In accordance with an embodiment of the present invention, an a-Si:H sensor is modified to include the selectable capacitor load by providing a third plate below the lower (second) plate of the sensor. The third plate is etched from the same metal layer used to form the gate line for accessing the select transistor of the pixel, thereby minimizing the number of additional process steps required to provide the selectable capacitor load. The third plate is selectively coupled to a ground line during high-capacitance operations (e.g., radiography) by a thin film transistor (TFT).